Evolution of ANB and SN-GoGn angles during craniofacial growth: A retrospective longitudinal study

INTRODUCTION

Sagittal malocclusions are highly prevalent^[1] and have functional, esthetic, and social implications^[2,3] that make them a public health issue. Their prognosis and clinical expression at adulthood are determined by facial growth.

Facial growth is a complex phenomenon characterized by the differential development of the jaws.^[4] Therefore, the sagittal intermaxillary relationship varies gradually between an early age and adulthood where normal young individuals have a greater facial convexity than adults.

A cephalometric measurement routinely used to diagnose sagittal skeletal dysplasias is the ANB angle.^[5,6] Its use is widespread among orthodontists because of its ease of use and its consistency with the clinical presentation in most cases. It also performs well as a diagnostic test, with high specificity and sensitivity assessing sagittal intermaxillary discrepancies.^[7-9]

Longitudinal cephalometric studies using historical records of North American individuals have reported that maxillary and mandibular sagittal development takes place with increases in SNA and SNB angles throughout growth, while ANB decreases.^[10-14] This ANB angle reduction has been reported to be significant in individuals between 7 and 15 years of age^[15] and has been consistently reported in different studies as a result of a relative dominance of sagittal mandibular growth.^[4,10-17] It is, thus, expected that ANB angle may diminish during development.^{[10,11,13,15,16,18-20]}

Clinically, the ANB angle is frequently used considering an average value of 2° and a standard deviation of 2°.^[6] However, this diagnostic approach does not take into account the maturational status of an individual, nor does it consider the possibility of a variation in the sagittal intermaxillary relationship during facial development. An early assessment, which fails to consider the above-mentioned developmental pattern, could incorrectly classify an individual, eventually leading to a wrong diagnosis and orthodontic treatment plan. Further, even knowing that a reduction in ANB is expected with growth, the amount of this reduction remains unclear. This is highly relevant in growing patients who carry Class II or Class III malocclusions.

Vertical development is also relevant in the sagittal plane. The maxillomandibular complex usually displays a counterclockwise rotation during the development.^[18] The SN-GoGn angle allows for the identification of variations between the mandibular plane and the cranial base.^[6,21] Its serial use can record the mandibular vertical developmental trend as it identifies the direction of mandibular growth rotation.

Maturational status is a key factor in facial growth evaluation. Different studies have assessed chronological age,^[10,16] which is a less desirable indicator than skeletal maturation, either analyzed in hand-wrist X-rays^[18,22] or at the cervical vertebrae.^[23] Given the progressive nature of development, the cephalometric values of ANB and SN-GoGn angles, analyzed relative to individual maturation, can provide useful information about dynamic phenomena in facial development, orthodontic diagnosis, and treatment planning.

The aim of this study is to describe the evolution of the ANB and SN-GoGn angles during the development, in a longitudinal sample of Caucasian patients classified according to the latest postpubertal ANB angle available and grouped by cervical vertebral maturation (CVM) stage.

MATERIALS AND METHODS

Historical cephalometric records from North American craniofacial growth centers were used to carry out an exploratory longitudinal study. The data for this study were obtained from the American Association of Orthodontists Foundation (AAOF) craniofacial legacy growth collection files, available on the internet. This collection contains radiographic longitudinal records of Caucasian individuals from ten growth centers in North America (Bolton-Brush, Burlington, Denver, Fels Longitudinal, Forsyth Twin, Iowa, Mathews, Meharry, Oregon, and Michigan Growth Studies).

AAOF craniofacial legacy growth collection files were reviewed for longitudinal records of individuals with pre- and post-pubertal lateral cephalometric X-rays, regardless of their sagittal classification. Clear lateral cephalometric radiographs in occlusion of orthodontically untreated male and female individuals with pre- and post-pubertal records were included in the study with three or more longitudinal cephalometric records. At least one postpubertal radiograph in vertebral cervical maturation (CVM) stage 5 or 6 had to be available. Following the inclusion criteria described above, from a total of 134 individuals and 1362 radiographs belonging to the online database with open access of the AAOF in August 2010, 71 individuals were selected for the study.

Longitudinal records were divided by gender and classified according to their skeletal developmental status using the CVM Method according to Baccetti et al.,^[23] [Table 1] with six maturational stages (cervical vertebral stage [CVS]). If one individual had several lateral radiographs displaying the same cervical maturation stage, the latest radiograph was selected to represent that given stage of maturation.

Cephalometric measurements were performed by a calibrated researcher using morphometric software (SigmaScan Pro 5.0, SPSS Science, Chicago, USA). After identifying points Sella (S), Nasion (N), A, B, Gonion (Go) and Gnation (Gn), ANB, SNA, SNB and SN-GoGn angles were measured in each radiograph for every individual. The individuals were classified based on their maxillomandibular sagittal relationship, according to the ANB angle at the latest postpubertal record available (CVS 5 or 6). Three groups were established, with Class I defined as those individuals with an ANB angle between 0 and 4 degrees. Values above or below this range were included in the Class II and Class III groups, respectively. Cephalometric angles were then recorded for every individual at each stage of skeletal maturation.

RESULTS

The sample included 71 individuals between 6- and 18-year-old, totaling 343 lateral radiographs (46% female), as shown in Table 2.

CVM progressed with increasing chronological age. No statistically significant (SS) differences were observed between genders (t-test P > 0.05). Data from males and females were grouped according to skeletal maturation and then analyzed together [Table 2].

Cephalometric measurements

Table 3 summarizes the descriptive variables for each group of CVM and divides them according to final skeletal class. Forty-five individuals were classified as skeletal Class I at the end of growth, 17 as Class II, and 9 as Class III.

Table 3 ANB, SNA, SNB, SN-GoGn angles description according to cervical vertebral maturation for each skeletal class

CVS	Class I					Class II					Class III
	ANB	SD	Minimum	Maximum	n	ANB	SD	Minimum	Maximum	n	ANB	SD	Minimum	Maximum	n
1	3.9	1.8	1.1	7.9	37	5.8	1.8	3.7	9.0	16	1.4	1.7	−0.8	3.3	6
2	3.4	1.4	0.9	7.3	41	5.9	1.6	3.2	8.2	15	−0.4	1.4	−2.6	2.1	7
3	3.0	1.3	0.6	6.4	40	5.7	1.7	3.1	8.5	16	−0.7	1.3	−2.4	0.7	4
4	2.8	1.3	0.8	5.1	34	5.3	1.6	2.9	7.9	14	−1.8	1.4	−3.1	0.2	4
5	2.4	1.3	0.1	5.0	36	5.4	1.3	3.4	7.5	13	−1.4	1.5	−4.2	0.1	9
6	2.4	1.2	0.0	3.9	35	5.0	0.9	4.2	7.5	14	−1.7	1.4	−3.8	−0.5	6
	SNA	SD	Minimum	Maximum	n	SNA	SD	Minimum	Maximum	n	SNA	SD	Minimum	Maximum	n
1	80.4	3.6	73.1	88.7	37	81.0	3.1	75.0	85.0	16	77.0	3.0	72.3	81.7	6
2	80.6	4.0	72.8	88.4	41	81.5	2.9	76.3	84.9	15	76.1	3.3	71.3	80.7	7
3	80.7	3.5	72.3	88.3	40	81.8	3.2	76.6	85.7	16	76.1	2.9	73.0	80.0	4
4	81.5	3.6	73.5	90.2	34	82.1	3.2	75.7	85.9	14	76.9	3.0	74.3	80.9	4
5	81.5	3.2	73.9	87.8	36	82.6	3.6	74.8	87.5	13	78.5	3.3	74.5	84.9	9
6	81.6	3.5	73.7	87.0	35	82.0	3.0	76.5	86.5	14	77.5	4.3	73.3	84.9	6
	SNB	SD	Minimum	Maximum	n	SNB	SD	Minimum	Maximum	n	SNB	SD	Minimum	Maximum	n
1	76.6	3.1	70.4	82.6	37	75.2	2.6	70.8	78.8	16	76.0	2.3	73.3	79.1	6
2	77.2	3.4	70.5	84.8	41	75.6	2.4	71.3	79.3	15	76.5	2.6	72.6	80.5	7
3	77.8	3.1	70.2	84.1	40	76.2	2.6	72.0	80.0	16	76.9	2.5	73.8	70.2	4
4	78.7	3.2	71.5	85.8	34	76.8	2.7	71.7	79.5	14	78.6	2.4	76.0	80.8	4
5	79.0	3.0	72.4	84.3	36	77.3	3.0	71.4	81.9	13	79.9	3.2	75.9	86.3	9
6	79.3	3.3	72.0	83.7	35	76.0	3.2	70.4	81.2	14	79.2	4.6	76.0	87.8	6
	SNGoGn	SD	Minimum	Maximum	n	SNGoGn	SD	Minimum	Maximum	n	SNGoGn	SD	Minimum	Maximum	n
1	33.8	5.0	25.5	47.1	37	34.8	4.3	28.3	39.6	16	31.7	4.0	26.1	38.6	6
2	33.1	4.9	23.2	44.2	41	34.3	4.4	28.5	39.5	15	31.0	2.0	28.3	34.4	7
3	31.8	4.5	21.4	41.2	40	33.5	4.5	27.7	40.3	16	30.3	2.6	27.6	33.4	4
4	30.7	4.4	21.4	40.4	34	33.1	4.5	27.6	40.0	14	28.5	2.1	25.4	30.1	4
5	30.5	5.3	19.9	44.8	36	31.5	4.4	27.0	41.1	13	28.3	3.2	25.0	33.9	9
6	29.8	5.2	17.2	39.7	35	32.3	4.8	26.0	41.2	14	26.8	2.6	24.5	30.3	6

SD – Standard deviation; n – Number of radiographs; CVS – Cervical vertebral stage

ANB values are in a range between −4.2° and 9.0°. It was observed that ANB values tend to decrease as growth occurs in every group. For Class I, an average ANB decrease of 1.5° was observed between the stages CVS 1 and 6. In Class II, the reduction was 0.7°, and 3.1° in Class III individuals [Table 3 and Figure 1].

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Figure 1

ANB angle description according to cervical maturation by skeletal class

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Both SNA and SNB angles increase as growth occurs while the SN-GoGn angle decreases [Table 3]. For the SNA angle, the highest values were recorded for Class II patients and the lowest for those in Class III [Table 3]. For angle SNB, the values are similar between Class I, II, and III. The lowest values are observed for Class II individuals [Table 2].

For the SN-GoGn angle, the values were similar between skeletal classes in the same CVS [Table 3]. A constant reduction of this value was observed as skeletal maturation increased. The largest decrease in the SN-GoGn measurement was seen in Class III patients, with an average reduction of 4.9°, in relation to a reduction of 4° for Class I, and 2.5° for Class II.

DISCUSSION

The present study was conducted to describe maxillomandibular relationships of clinical relevance throughout the development in a longitudinal sample of North American Caucasian individuals, obtained from different growth centers. The individuals were classified by maturational status and grouped by sagittal class. To account for the different radiographic magnification factors, only angular measurements were carried out, which allowed for the identification of relevant aspects of maxillomandibular development.

It is known that significant craniofacial differences may be observed between Caucasian individuals from different growth centers.^[10] Several studies have grouped individuals according to chronological age and correlated them with skeletal changes.^[24-29] However, if a heterogeneous sample of individuals is grouped considering sagittal and maturational characteristics, the inclusion of individuals from different growth centers may strengthen the external validity of the results. That was the approach adopted in the present study. The results are therefore grouped according to skeletal maturity and not to chronological age, making the subgroups of individuals comparable. The determination of skeletal maturity provides a better basis of comparison than chronological age for grouping individuals.^[30]

Sexual dimorphism is clearly expressed in facial development,^[22,31,32] mainly as differences in linear cephalometric measurements among genders.^[27,33] The present study found no differences between sexes during growth using angular measurements. It is likely that the grouping methodology using skeletal maturation rather than age, together with the absence of linear measurements, may have influenced these findings.

The results of this study indicate that ANB angle decreases during growth, which is consistent with the results of other studies.^{[10-16,30,34]} In the present study, the reduction in the ANB angle during development took place in all groups but differed according to skeletal class, with decreases averaging 1.5° in Class I, 0.85° in Class II, and 3.1° in Class III. This may imply a differential expression of maxillomandibular development dependent on the skeletal class. The smallest average variation occurred in Class II patients while the opposite was seen in Class III. Class III individuals experienced a greater decrease in the ANB angle than Class I, expressing their sagittal condition at a later point of development, as previously reported.^[33] This may partially explain the progressive nature of the severity of Class III malocclusions. It also may account for the improvement in the sagittal relationship occasionally observed in patients with large ANB angle values at early ages that evolve to normal sagittal relationships as shown in Table 3. In fact, the Class I group included individuals with ANB values above 7° in CVS 1, which decreased to the Class I range in CVS 6.

Another factor that may affect the ANB angle reduction with age is mandibular counterclockwise rotation,^[16] which is consistent with the decrease in the angle SN-GoGn observed in this sample. Thus, both the dominance of mandibular growth over the maxilla during development and a counterclockwise mandibular rotational tendency contribute to a reduction in facial convexity that is generally expected during facial development. The magnitude of these phenomena and their clinical relevance, aside from the skeletal classification, may relate to interindividual variability.

The SN-GoGn angle is an angular measurement included in the study to quantify the inclination of the mandibular base relative to the cranial base. Its average value is 32°.^[6] Subranamian and Naidu^[30] found a decrease from 36° to 31° between 6 and 16 years of age. This correlates with the findings in the present study, where there is an average decrease between CVS 1 (average 8-year-old) and CVS 6 (average age 17), from approximately 34°–30° in Class I individuals. Class II and III individuals also displayed a reduction throughout growth, with a 2.5° and 4.9° reduction, respectively, showing a differential tendency among groups, consistent with the one observed with the ANB angle.

In the analyzed sample, both SNA and SNB angles increase as the individuals grow, with a higher increase in the SNB angle. The same has been described by others.^[10,27] The SNA angle measurements in Class I and II individuals are very similar, and the differences between the skeletal classes originate in the mandibular position; therefore, Class II individuals had more retrognathic mandibles than did Class I as Jacob and Buschang^[35] have reported. Others, however, have reported a greater difference between Class II and Class I^[24] with the SNA angle 2–3° higher in Class II than in Class I individuals. On the other hand, in the present study, Class III individuals displayed a smaller SNA angle during development and similar SNB values between Class I and Class III individuals, while it was lower in Class II individuals. Reyes et al.,^[36] analyzing an extensive cross-sectional sample of untreated Class III individuals, have found no differences between Class III and Class I individuals for the sagittal position of the maxilla in any of the age ranges, and larger SNB values in the Class III sample. Class II and III malocclusions are etiologically diverse and may result from altered mandibular or maxillary positions, or a combination of both. Our Class II and III groups included 17 and 9 untreated individuals, respectively, with sagittal malocclusions of a moderate severity, which were followed longitudinally. It is, thus, a small sample that reflects the growth tendency of individuals with sagittal malocclusions due to mandibular and maxillary deficiency in Class II and III individuals, respectively. This group may differ from others in their severity and reflects the diversity of expression that characterizes sagittal malocclusions.

CONCLUSIONS

ANB and SN-GoGn angles decrease during growth whereas SNA and SNB increase.

The magnitude of the decrease in ANB and SN-GoGn angles during growth varies depending on individual sagittal characteristics. They decrease more in Class III individuals and less in Class II individuals.

It is expected that regardless of the sagittal classification of an individual, the value of the measure of ANB and SN-GoGn angles tends to decrease during development.

Class II individuals presented a reduced SNB angle while Class III individuals displayed a reduced SNA angle.